Al203 and zro2 based abrasive grain, method for the production and use thereof

ABSTRACT

An Al 2 O 3  and ZrO 2  based abrasive grain containing titanium compounds in a reduced form, especially in the form of oxides and/or suboxides and/or carbides and/or oxycarbides and/or oxycarbonitrides and/or suicides, corresponding to a proportion of 0.510 wt. %, expressed as TiO 2 , containing 20-50 wt. % ZrO 2 , wherein more than 75 wt. % of the ZrO 2  is present in a tetragonal crystal form, having an overall carbon content of 0.03-0.5 wt. %, a proportion of impurities caused by raw materials of less than 3.0 wt. %, wherein the rare earth content calculated in the form of oxides is less than 0.1 wt. %, having an Si-compound content of 0.05-1.0 wt. %, expressed as SiO 2 , and an Al 2 O 3  content of 50-80 wt. %, obtained by melting a mixture required for a corresponding abrasive grain composition, said mixture corresponding to Al 2 O 3 , ZrO 2 , TiO 2  and TiO 2 , and/or raw materials containing the latter in the presence of a surplus of a reducing agent and quenched in such a way that the melt is fully solidified in less than 10 seconds. The invention also relates to a method for the production thereof and the use thereof in abrasives.

The invention at hand relates to an abrasive grain on the basis of Al₂O₃ and ZrO₂ with a contents of titanium compounds in reduced form, in particular in the form of oxides and/or suboxides and/or carbides and/or oxycarbides and/or oxycarbonitrides and/or suicides between 0.5 and 10 wt. %, expressed as TiO₂; a contents of ZrO₂ between 20 and 50 wt. %, with more than 75 wt. % of the ZrO₂ being present in its tetragonal crystal form; an overall carbon contents between 0.03 and 0.5 wt. %; a portion of raw-material caused impurities of less than 3.0 wt. %, with the contents of rare earths calculated as oxides amounting to less than 0.1 wt. %; a contents of Si compounds between 0.05 and 1.0 wt. %, expressed as SiO₂; and a contents of Al₂O₃ between 50 and 80 wt. %, obtainable by melting a mixture necessary for the corresponding composition of the abrasive grain of Al₂O₃, ZrO₂, TiO₂ and SiO₂ and/or of raw materials containing the latter, in the presence of an excess of a reducing agent, and quenching it in such a way that the melt fully solidifies in fewer than 10 seconds.

Abrasive grains on the basis of Al₂O₃ and ZrO₂ produced through the extremely rapid cooling of a melt of Al₂O₃ and ZrO₂ have been known for approximately 40 years. In selected grinding processes they distinguish themselves through particularly advantageous abrasive properties.

For example, a zirconium corundum abrasive grain is described in U.S. Pat. No. 3,981,408 that is cooled down extremely fast by pouring a melt of Al₂O₃ and ZrO₂ produced under reducing conditions between metal plates which prevents a segregation of the components and which permits the stabilization of an extremely fine crystallite structure and, at the same time, of a certain portion of tetragonal high-temperature modification of the ZrO₂. Due to the microcrystalline structure, the zirconium corundum produced in this fashion is particularly well suited for use in abrasives since, on the one hand, it is of high toughness and thus of great sturdiness, and, at the same time, it has a tendency, under the corresponding high stress during the grinding process, to form new cutting edges through the breaking off of smaller segments from the abrasive grain which is fostered and made possible through the microcrystalline structure. These new cutting edges in turn intervene in the grinding process which leads to a cool and thereby material-friendly grinding as well as a high abrasion yield. According to U.S. Pat. No. 3,891,408, the contents of SiO₂ should be as low as possible. TiO₂ is viewed as less damaging.

In the past, it has been tried again and again to further improve the performance of the zirconium corundum abrasive grains through diffusions and variations of the quenching process and the degree of reduction.

For example, a zirconium corundum is described in DE-A-3 040 992 with a portion of ZrO₂ of 27 to 35 wt. % which also contains, expressed as TiO₂, 1 to 10 wt. % of oxycarbides, carbides and/or suboxides of titanium which are obtained through reduction of TiO₂ with coal during the melting process. The abrasive performance of this abrasive grain is characterized as equal or superior to that of the abrasive grain described in U.S. Pat. No. 3,891,408. However, the advantage of this abrasive grain is seen, above all, in the lowering of the production costs while providing a comparable abrasion performance which is obtained through a reduction of the ZrO₂ portion. In DE-A-3 040 992, the assumption is expressed that the direct influence on the grinding process by the Ti compounds generated by the reduction is responsible for the improvement of the properties of the abrasive grain.

In U.S. Pat. No. 5,143,522 an abrasive grain is described, with a contents of ZrO₂ of 20 to 50 wt. %, of which more than 25 wt. % are present in its tetragonal form, 1 to 10 wt. % of reduced titanium oxide in the form of suboxides, carbides and oxycarbides, 0.3 to 0.5 wt. % carbon, less than 3 wt. % of impurities, and Al₂O₃ as its main component. The structure of the abrasive grain is described as deposits of primary Al₂O₃ crystals in a eutectic aluminum oxide/zircon oxide matrix. The size of the primary crystals lies between 5 and 50 μm. The share of ZrO₂ in the tetragonal crystal form amounts to up to 75 wt. % relative to the overall contents of ZrO₂. Viewed as harmful impurities are, above all, Na₂O, whose contents should not exceed 0.1 wt. %, and SiO₂, whose contents should not exceed 1 wt. %, while ideally lying below 0.1 wt. %. Other raw-material related impurities mentioned are MgO, CaO and Fe₂O₃, whose contents should not exceed 1.5 wt. %.

Contrary to the above-mentioned publication and to U.S. Pat. No. 3,891,408 as well as to the general opinion of the experts, GB-A-2 011 880 describes a zircon corundum on the basis of bauxite and baddeleyite containing up to 2.5 wt. % TiO₂ which contains between 1.1 and 2.0 wt. % SiO₂ and which is said to be advantageous for certain uses vis-á-vis the materials made from purer raw materials. The essential difference with all other processes described here is the fact that inexpensive bauxite is used directly as the raw material. However, when using bauxite as a raw material for zircon corundum, large amounts of impurities need to be reduced (bauxite always contains, above all, larger amounts of iron mineral, in addition to titanium compounds and quartz or, respectively, silicates). In contrast with DE-A-3 040 992, with U.S. Pat. No. 5,143,522 and even with EP-B-0 595 081 mentioned later, in the process according to GB-A-2 011 880 the coal for the reduction of the impurities is used up, presumably leading to the formation of an alloy of metallic iron and silicon. The high contents of more than 1% SiO₂ in the products described points at the fact that the work was done with relatively small amounts of coal as a reduction agent, which means that a major difference with the publications described above lies simply in the fact that presumably titanium is not present in the product in its reduced forms that are responsible for the stabilization of the tetragonal ZrO₂ modifications and which have abrasion-active properties.

In U.S. Pat. No. 4,457,767 a zircon corundum abrasive grain is described that contains between 0.1 and 2% of yttrium oxide which serves to stabilize the tetragonal high-temperature modification. In contrast with the stabilization with TiO₂, our own tests have shown that the viscosity of the melt is influenced little or not at all through the addition of Y₂O. In comparison with the melts in which TiO₂ is used as stabilizer for the tetragonal ZrO₂ modification, this provides the advantage that a rapid quenching of the melt and the realization of an extremely fine structure is possible without any hindrance. However, this disadvantage for the Ti-containing zircon corundum is compensated for by the abrasion-active effect of the reduced Ti compounds.

Japanese patent JP-A-1 614 974 describes a titanium-containing zircon corundum that also contains Y₂O₃ with which the high-temperature modification of the Y₂O₃ is stabilized.

In EP-B-0 595 081 on which the invention is based an abrasive grain is described with a contents of titanium compounds in the form of suboxides, carbides and/or oxycarbides in which more than 90 wt. % of the ZrO₂ are present in its tetragonal modification and which is obtained by quenching the melt to complete solidification in fewer than 10 seconds. Stabilization of the tetragonal high-temperature modification of the ZrO₂ is reached in this case without any addition of Y₂O₃ and only through stabilization with the aid of the reduced titanium compounds and through rapid quenching.

Methods for a quick quenching of oxide melts are described, for example, in U.S. Pat. No. 3,993,119, U.S. Pat. No. 4,711,750, and in EP-B-0 593 977.

All methods and materials described above had as their objective to satisfy the demand by users for an abrasive grain with even higher abrasion performances and more favorable abrasion behavior, in particular for the treatment of various kinds of steel. While the methods and materials mentioned have proved their mettle in the past, they can not meet the increased demands of the manufacturers of abrasives or, respectively, of their clients, for further performance improvements and an even more efficient use of the abrasives.

The task of the invention is therefore to make available a zircon corundum abrasive grain that for the treatment of a wide variety of materials, in particular of the most varied kinds of steel, represents an improvement in terms of performance as compared with the state of the art.

This task is solved by providing an abrasive grain on the basis of Al₂O₃ and ZrO₂ with a contents of titanium compounds in a reduced form, in particular in the form of oxides and/or suboxides and/or carbides and/or suicides, between 0.5 and 10 wt. %, expressed as TiO₂; a contents of ZrO₂ between 20 and 50 wt. %, with more than 75 wt. % of the ZrO₂ being present in its tetragonal crystal form; with an overall carbon contents of between 0.03 and 0.5 wt. %; a portion of raw-material caused impurities of less than 3.0 wt. %, with the contents of rare earths calculated as oxides amounting to less than 0.1 wt. %; a contents of Si compounds between 0.05 and 1.0 wt. %, expressed as SiO₂; and a contents of Al₂O₃ between 50 and 80 wt. %, obtainable by melting a mixture necessary for the corresponding composition of the abrasive grain of Al₂O₃, ZrO₂, TiO₂ and SiO₂ and/or of raw materials containing the latter, in the presence of an excess of a reducing agent, and quenching it in such a way that the melt fully solidifies in fewer than 10 seconds.

The abrasive grain distinguishes itself through the fact that abrasives manufactured with this abrasive grain yield increases in the abrasion performance of up to 50% and more in the treatment of a wide variety of materials, in particular of the most varied kinds of steel, as compared with the known abrasives on the basis of zircon corundum, under comparable and/or the same test parameters.

In preferred model versions of the abrasive grain according to the invention as shown in claims 2 through 14, the contents of reduced titanium compounds lies between 2 and 5 wt. %, and/or the contents of tetragonal ZrO₂ modification at not less than 85 wt. %, especially preferably at not less than 90 wt. %, and/or the contents of impurities below 1 wt. %, and/or the contents of Si compounds between 0.2 and 0.7 wt. %.

Surprisingly, it was found that, in analogy with EP-B-0 595 081, during the manufacturing of an abrasive corundum on the basis of Al₂O₃, and ZrO₂, an abrasive grain can be obtained through the purposeful addition of SiO₂-containing compounds during the melting process that is clearly superior to the zircon corundum known so far.

This was all the more surprising since during the manufacturing of abrasive grains on the basis of corundum, SiO₂ impurities are usually avoided because the silicates formed during the melting process do not possess any outstanding grinding properties and thus reduce the performance of the abrasive grain. In the production of zircon corundum abrasive grains, too, the general trend among experts was to employ the purest possible raw materials and to keep the contents of impurities as low as possible. Only for cost reasons or for reasons of availability did one resort to more strongly contaminated raw materials. Therefore, the zircon corundum and abrasive grains themselves that are available on the market are relatively pure products; however, as a rule they contain between 0.02 and 0.3 wt. % SiO₂; but a dependence of the performance on the SiO₂ contents can not be gleaned from the relevant underlying publications; instead, it is—as mentioned at the onset—regarded as rather detrimental. In most commercial products the contents lies at approximately 0.2 wt. %. Only the product made by the registrant in accordance with EPB-0 595 081 shows a clearly lower contents of SiO₂, which lies at approximately 0.02 wt. %. A correlation between SiO₂ contents and the performance potential of the abrasive grain is not indicated in EP-B 0 595 081.

In the production process of the abrasive grain, for example, a mixture of 50 to 80 wt. % of Al₂O₃, preferably alum earth, and 20 to 50 wt. % ZrO₂ in the form of baddeleyite, zircon concentrate and/or zircon sand in the presence of 0.5-10 wt. % TiO₂ which is preferably used in the form of rutile, and 0.5 to 5 wt. % SiO₂ which is preferably added to the mixture in the form of zircon sand, is melted in the electric arc furnace under reducing conditions in the presence of 0.5-5 wt. % coal. The molten mixture is then quenched as quickly as possible, which can be achieved by pouring the melt in accordance with EP-B-0 593 977 into a narrow fissure between metal plates, with the liquid material completely solidifying within a few seconds, preferably in fewer than 5 seconds, and particularly preferably in fewer than 3 seconds. The gap between the metal plates that is formed by corresponding recesses in metal plates assembled to a plate packet, has a width of 1 to 10 mm, preferably 2 to 5 mm. The cooling plate packet that during the pouring of the melt is guided with a defined speed below the pouring lip of the furnace vessel has a starting temperature of approximately 100 to 500° C. Following the pouring, the temperature of the cooling plate packet has increased by approximately 50-100° C. due to the filling of the gaps with 2000° C. hot melt, causing the cooling plate packet to be driven to a so-called cooling zone after being emptied, where it is sprayed with water to bring it back to the preset starting temperature. The cooling plate packet is then put in its starting position for the next pouring action which can be started as soon as enough melt is present in the furnace vessel.

However, in addition to the above-described process for the quenching of the liquid melt which corresponds to the method described in EP-B-0 593 977, any other suitable state-of-the-art method for a quick quenching of liquid oxide melts may be used, for example the methods described in US patents U.S. Pat. No. 3,993,119 and U.S. Pat. No. 4,711,750.

For the production of the abrasive grain in accordance with the invention, alum earth, zircon concentrate, zircon sand and rutile are used preferably as raw materials, with the raw-material mixture preferably being chosen in such a way that the end product contains a preponderance of a eutectic mixture of ZrO₂ and Al₂O₃, with an Al₂O₃ contents of approximately 55-65 wt. %, a ZrO₂ contents of approximately 35-45 wt. %, each relative to the overall contents of Al₂O₃ and ZrO₂.

The carbon contents of the product lies between 0.03 and 0.5 wt. %, while between 0.5 and 5 wt. %, preferably 1 to 4 wt. %, are used in the starting mixture. That means that a large portion of the coal is used up during the melting process for the stabilization of the reducing conditions in the melt, for the reduction of the TiO₂ and for the reduction of the SiO₂, escaping during this process to a large part as CO. The relatively low contents of SiO₂ in the product, with comparatively large amounts in the starting mixture, indicates that under the chosen reducing conditions SiO₂ is reduced to a large part to SiO and vaporized, and only a small portion is reduced to silicides and/or other Si compounds. Any reduction to metallic Si that should then be deposited at the bottom of the furnace together with other metallic impurities that were formed from the oxides through reduction in the raw material will be low.

As in EP-B-0 595 081, the Ti compounds, possibly suboxides or other reduction products of titanium formed during the reduction, cause a stabilization of the tetragonal high-temperature modification of the ZrO₂, leading, in combination with the above-described effective quenching of the melt, to a very high contents of tetragonal ZrO₂ modification in the product.

However, upon a comparison with materials that are identical with the abrasive grain in accordance with the invention, down to the SiO₂ contents in the melt and in the end product, and that are melted and quenched in accordance with the same method, it becomes conspicuous that with the use of TiO₂ for the stabilization of the tetragonal ZrO₂, the addition of SiO₂ clearly behaves counterproductively for a stabilization of the tetragonal ZrO₂ modification, and that the product in accordance with the invention, with otherwise the same starting mixture, shows a lower contents of tetragonal ZrO₂ modification than a comparable abrasive grain with a lower SiO₂ contents. EP-B-0 595 081 now teaches us that the best abrasion results are achieved with abrasive grains that have a tetragonal ZrO₂ modification contents of at least 90 wt. % relative to the overall ZrO₂ contents. It was all the more surprising that in spite of a comparatively reduced ZrO₂ modification contents, clearly better results could be achieved with the abrasive grains according to the invention for all areas of application that were tested than with the corresponding state-of-the-art abrasive grains that showed a higher contents of tetragonal ZrO₂ modification. As a matter of principle, however, it is true also with respect to the abrasive grain in accordance with the invention that a high contents of tetragonal phase will yield abrasion-technological advantages. For example, the portion of tetragonal phase may be increased again through the addition of TiO₂ which indicates that it is possible that initially at least part of the SiO₂ reacts in the melt with TiO₂, e.g. to form Ti silicides, thereby lowering the share of the reduced Ti compound(s) that is (are) responsible for the stabilization of the tetragonal ZrO₂ modification. By increasing the TiO₂ share, this portion can be increased again. In doing so, it comes as a surprise that it is possible to increase the contents of tetragonal ZrO₂ phase again, without increasing the viscosity of the melt to such a degree that the quenching performance of the melt deteriorates again.

An explanation of this behavior may be found in the physical properties of the melt itself which is altered through the presence of SiO₂. Thus, it can be seen that the viscosity of the melt which increases through the formation of poorly soluble and high-melting titanium compounds in the course of the melting process under reducing conditions, with an increasing titanium oxide contents, can be lowered again by adding SiO₂ which, e.g., facilitates the pouring of the liquid melt into a relatively narrow gap between metal plates. The faster the liquid melt can be poured between the cooling plates, the faster the quenching of the melt will proceed, and the finer the structure that can be stabilized in the product.

Structure tests carried out on the abrasive grain according to the invention in accordance with examples 1 through 10 and comparative examples 1 and 2 speak in favor of the theory that, in addition to a high contents of tetragonal ZrO₂ phase and the presence of titanium compounds in the form of oxides and/or suboxides and/or carbides and/or oxycarbides and/or oxycarbonitrides and/or silicides, it is, above all, the structure of the abrasive grain that is responsible for its performance potential. The finer the structure, the more effective will be the corresponding abrasive grain. The structure tests were carried out by means of back scatter images with a scanning electron microscope and by way of polished grindings. It turned out that with the optimally cooled down materials, the eutectic mixture of Al₂O₃ and ZrO₂, has such a fine structure even at 10,000-fold magnification that the size of the individual precipitations which lies below 0.1 μm can no longer be detected accurately via back scatter electron images in a scanning electron microscope. But what is clearly recognizable in all tested samples are the individual dendritic eutectic areas which have a domain-like configuration, with the cross section of the dendritic eutectic domains decreasing in tandem with the decrease in the size of the individual precipitations of Al₂O₃ and ZrO₂. The dendritic eutectic domains themselves lie within a magnitude of a few micrometers and more and can therefore be measured by means of electron microscopic images and be used for the characterization of the structure of the abrasive grains. Corresponding measurements were carried out within the framework of these works during which it was detected that the median cross section of the measured dendrites of all reference examples lies below 20 μm, and of the preferred versions below 15 μm, and that they thus have a clearly finer structure than comparative example 1 which was melted without the addition of SiO₂. The results have been summarized in Table 2.

From the grinding tests summarized in Tables 3 through 5 it can be deduced that the performance potential of the abrasive grain increases when the structure becomes finer and finer. At the same time, it has been known from EP-B-0 595 081—and it can be repeated in accordance with the grinding tests—that a high contents of tetragonal ZrO₂ phase has a favorable effect on the performance potential; however, this effect stays within limits in the examples at hand since all examples cited already have a relatively high contents of tetragonal phase.

The objective of the invention is therefore solved by finding the optimal combination of Ti compounds and Si compounds for the production of a high-quality zircon corundum abrasive grain.

The effect of the titanium compounds on the grinding performance is of a complex nature. For example, TiO₂ is initially used for the stabilization of the tetragonal ZrO₂ phase. However, the decisive factor in this is the fact that the work is carried out under reducing conditions since tests without any reduction agents proved that one or several of the reduced titanium compounds is/are obviously responsible for the stabilization of the tetragonal ZrO₂ phase, which can also be seen in example 3 in EP-B-0 595 081 in which, among others, zircon corundum melt with added titanium oxide was quenched without the addition of any reduction agent. Corresponding tests that confirmed this result were also carried out within the scope of this work. The question of what titanium compounds are ultimately responsible for the stabilization is the subject of further studies that have not yet been concluded. But it is quite obvious that the titanium compounds that are formed during the melting process under reducing conditions in the presence of coal, such as titanium carbide, titanium oxycarbide, titanium carbonitride, and others, also have a positive influence on the abrasion performance which could possibly be explained by the great hardness of those compounds. However, in addition to that, the extremely poorly soluble and high melting titanium compounds alter the viscosity of the melt and thereby the cooling behavior of the melt as well. A melt with a high viscosity can, e.g., be poured only with difficulty into a narrow gap between metal plates; in addition, the coating of the metal with liquid melt deteriorates, which in turn hampers the heat transfer und thereby a quick cooling. The addition of SiO₂ reduces the viscosity of the melt, but at the same time—possibly due to the formation of titanium silicides—the equilibrium of the titanium compounds being formed is shifted, which obviously affects the compound responsible for the stabilization of the tetragonal SiO₂ phase. An increase of the TiO₂ contents allows to shift the equilibrium again in such a way that high portions of tetragonal ZrO₂ phase are possible again, surprisingly without increasing the viscosity of the melt to such a degree that the quality of the quenching of the melt or, respectively, the structure of the product deteriorates. While a further increase of the TiO₂ portion permits the stabilization of even higher portions (100%) of tetragonal ZrO₂ phase, the viscosity of the melt is simultaneously increased to such a degree that overall a deterioration of the product results due to the coarser structure which in turn may be attributed to the inferior cooling conditions.

Another and/or additional possible explanation for the surprisingly good abrasion results is the formation of a glass phase in the abrasive grain which increases the wear resistance of the abrasive grain in accordance with the invention.

In the following the present invention is explained in detail, without this constituting any limitation.

EXAMPLE 1

A mixture of 335 alum earth (AC34, firm of Pechney, Gardanne), 232 kg of baddeleyite concentrate (firm of Foskor, South Africa), 16 kg of petrol coke, 16 kg of rutile, and 10 kg of zircon sand with a composition of 66.6% ZrO₂, 32.5% SiO₂, 0.32% Al₂O₃, 0.1% Fe₂O₃ and 0.14% of TiO₂ was melted in an electric arc furnace. The furnace vessel had a diameter of approximately 2 m, the voltage was 110 V, and the load was selected at 1,500 kWh. The mixture was charged in such a way that after approximately one hour the entire mixture had melted and could be quenched in accordance with a method as per EP-B-0 593 977. The starting temperature for the cooling plates amounted to approximately 300° C. and the gap between the metal plates had a width of 3 to 5 mm. Previous tests had shown that under these conditions the material completely solidified only 3 seconds after being poured between the cooling plates. Approximately 1 minute after the pouring, the plate packet reaches a device for evacuation in which the solidified and cooled down material is removed from the plate packet, and collected. At the time of the evacuation the solidified material still had a temperature of approximately 350° C. Following a complete cooling down, the material can be crushed in the usual manner (for example by jaw crushers, rolling crushers, roller mills or similar crushing aggregates) and subsequently be treated for graining by way of proper sifting.

EXAMPLES 2 THROUGH 10

The production of the zircon abrasive grains occurred in analogy with Example 1. However, the addition of zircon sand and, correspondingly, the portion of baddeleyite concentrate was varied. In addition, the rutile contents was varied as well. Table 1 shows the starting mixtures of zircon sand, baddeleyite concentrate and rutile necessary for the various model variations of the abrasive grain according to the invention shown in Examples 2 through 10. The amounts of alum earth and petrol coke were not altered as compared with Example 1 and are therefore not shown separately in the following table. TABLE 1 Starting mixtures for Examples 2-10 (without Al₂O₃ and petrol coke whose portions remained unchanged in all runs) Baddeleyite Abrasive Grain Zircon Sand Concentrate Rutile Example 2 20 kg 228 kg 16 kg Example 3 40 kg 228 kg 16 kg Example 4 80 kg 206 kg 16 kg Example 5 120 kg  195 kg 16 kg Example 6 40 kg 228 kg 22 kg Example 7 60 kg 214 kg 28 kg Example 8 80 kg 206 kg 22 kg Example 9 80 kg 206 kg 28 kg Example 10 80 kg 206 kg 40 kg

For a direct comparison with the state of the art, the comparative examples listed in the following were produced under the same conditions as Examples 1-10. In their chemical compositions, their physical properties and their abrasion-technological properties, the comparative examples thus correspond to the state-of-the-art materials with the best performance currently available on the market.

COMPARATIVE EXAMPLE 1

Production occurred in analogy with Example 1. However, no zircon sand was added. The starting mixture contained 335 kg of alum earth, 235 kg of baddeleyite concentrate, 16 kg of rutile and 16 kg of petrol coke.

COMPARATIVE EXAMPLE 2

Production occurred in analogy with Example 2. However, no rutile was added, instead, stabilization of the tetragonal ZrO₂ modification was achieved by adding 5 kg of Y₂O₃.

The chemical analyses of the most important components of the abrasive grains of the reference examples and of the comparative examples have been compiled in Table 2. The portions of Al₂O₃ and ZrO₂ were selected in each case in such a way that the mixture was as close to the eutectic system for Al₂O₃ and ZrO₂ as possible, meaning that for all examples, the contents of Al₂O₃ and ZrO₂ was nearly identical and in each individual case lay between 55 and 59 wt. % for Al₂O₃ and, respectively, between 39 and 42 wt. % for ZrO₂. Within these narrow limits, none of the possible variations in the share of the main components Al₂O₃ and ZrO₂ has any discernible impact on product quality, and they are therefore not listed in Table 2 either.

In addition, the portions of tetragonal ZrO₂ modification relative to the overall contents of ZrO₂ are listed in Table 2. The portions of tetragonal ZrO₂ modification is calculated according to the equation ${T\left( {{wt}.\quad\%} \right)} = \frac{2t \times 100}{{2t \times m_{1}} + m_{2}}$ which is based on the radio diffraction analysis of zircon corundum powders (radio powder diffractogram), with t denoting the intensity of the peak of the tetragonal modification at a diffraction angle of 2 theta=30.3, m₁ denoting the intensity of the peak of the monocline modification at a diffraction angle of 2 theta=28.3, and m₂ denoting the peak of the monocline modification at a diffraction angle of 2 theta=31.5. The intensity itself was determined via the integral of the peak area.

Annotations re Table 2 (see below): TABLE 2 Characterization of the various reference examples and comparative examples Structure (median cross section of the Share of dendritic tetragonal eutectic Abrasive Grain Chem. Analysis ZrO₂ modification domains)¹⁾ Example 1 TiO₂ = 2.45% 87% 12 μm SiO₂ = 0.08% Example 2 TiO₂ = 2.52% 84% 10 μm SiO₂ = 0.21% Example 3 TiO₂ = 2.50% 82% 11 μm SiO₂ = 0.32% Example 4 TiO₂ = 2.46% 78%  9 μm SiO₂ = 0.45% Example 5 TiO₂ = 2.56% 72%  9 μm SiO₂ = 0.78% Example 6 TiO₂ = 3.25% 91% 17 μm SiO₂ = 0.29% Example 7 TiO₂ = 4.26% 94% 19 μm SiO₂ = 0.32% Example 8 TiO₂ = 3.36% 89% 12 μm SiO₂ = 0.48% Example 9 TiO₂ = 4.26% 95% 12 μm SiO₂ = 0.49% Example 10 TiO₂ = 6.31% 100% 18 μm SiO₂ = 0.48% Comparative TiO₂ = 2.48% 93% 24 μm Example 1* SiO₂ = 0.02% Comparative TiO₂ = 0.08% 92% 12 μm Example 2** Y₂O₃ = 0.85% SiO₂ = 0.24% *Comparative example 1 corresponds to a commercially available, state-of-the-art zircon corundum in accordance with EP 0 595 081 B1. **Comparative example 2 corresponds to a commercially available, state-of-the-art zircon corundum in accordance with U.S. Pat. No. 4 457 767. ¹⁾The median cross section of the dendrites was determined by measuring back scatter electron images at 500-fold magnification of zircon corundum platelets with a median cross section of 3 mm that were sharpened lengthwise to the growth direction of the dendrites. The zircon corundum platelets themselves # were obtained by pouring the liquid melt into a gap between metal plates with a median gap width of 3 mm, with the growth of the dendrites proceeding vertically from the cooling plate into the interior of the poured and solidified zircon corundum platelet.

The abrasive grains listed in Table 2 were used to conduct grinding tests that are explained in detail below, and summarized in Tables 3 through 5. TABLE 3 Test results of belt grinders against work pieces ST 37 (grinding time 12 min) and V2A (grinding time 30 min), each with a P40 graining according to FEPA Grinding Test 1 Abrasion Performance Abrasive Grain Steel Pipe (ST37) V2A Surface Example 1 541 g = 115% 375 g = 108% Example 2 621 g = 132% 396 g = 114% Example 3 668 g = 142% 424 g = 122% Example 4 603 g = 134% 379 g = 109% Example 5 499 g = 106% 361 g = 104% Example 6 582 g = 124% 400 g = 115% Example 7 555 g = 118% 403 g = 116% Example 8 658 g = 140% 449 g = 129% Example 9 677 g = 144% 459 g = 132% Example 10 503 g = 107% 393 g = 113% Comparative Example 1 470 g = 100% 348 g = 100% Comparative Example 2 461 g = 98% 361 g = 104%

TABLE 4 Test results of belt grinders C45 and V2A (in each case, solid material, 30 mm diameter)/contact pressure 35 N/grinding time 50 minutes Grinding Test 2 Abrasion Performance Abrasive Grain Graining C45 V2A Example 1 P36 2953 g = 105%  890 g = 112% P50 2269 g = 114%  712 g = 109% Example 2 P36 3571 g = 127%  930 g = 117% P50 2428 g = 122%  888 g = 136% Example 3 P36 3684 g = 131% 1097 g = 138% P50 2527 g = 127%  966 g = 148% Example 4 P36 3599 g = 128% 1065 g = 134% P50 2607 g = 131%  803 g = 123% Example 5 P36 2896 g = 103%  779 g = 98% P50 2109 g = 106%  660 g = 101% Example 6 P36 3627 g = 129%  970 g = 122% P50 2666 g = 134%  728 g = 115% Example 7 P36 3065 g = 109%  890 g = 112% P50 2267 g = 114%  718 g = 110% Example 8 P36 4077 g = 145% 1097 g = 138% P50 2706 g = 136%  914 g = 140% Example 9 P36 3965 g = 141% 1167 g = 147% P50 2647 g = 133%  993 g = 152% Example 10 P36 2868 g = 102%  898 g = 113% P50 2249 g = 113%  764 g = 117% Comparative P36 2812 g = 100%  795 g = 100% Example 1 P50 1990 g = 100%  653 g = 100% Comparative P36 2792 g = 99%  783 g = 98% Example 2 P50 1908 g = 96%  705 g = 108%

TABLE 5 Test results of belt grinders SUS 304 (grinding time 5 min) and S 50 C (grinding time: 30 min)/P40 graining according to FEPA Grinding Test 3 Abrasion Performance SUS 304 Stainless Steel S 50 C Construction Steel Abrasive Grain High Stress Medium Stress Example 1 541 g = 123% 2120 g = 107% Example 2 621 g = 127% 2080 g = 105% Example 3 542 g = 138% 2311 g = 116% Example 4 603 g = 135% 2267 g = 114% Example 5 415 g = 104% 1941 g = 98% Example 6 495 g = 124% 2219 g = 112% Example 7 555 g = 108% 2020 g = 102% Example 8 547 g = 137% 2536 g = 128% Example 9 567 g = 142% 2833 g = 143% Example 10 431 g = 108% 2199 g = 111% Comparative 399 g = 100% 1981 g = 100% Example 1 Comparative 423 g = 106% 2056 g = 104% Example 2

As can be seen from the examples, increases of the abrasion performance of up to approximately 50% and more are achieved with abrasives produced with this abrasive grain in the treatment of a wide range of materials while maintaining comparable and/or the same test parameters as compared with the abrasives known so far on the basis of zircon corundum. This applies in particular to flexible zircon corundum abrasives such as grinding belts and abrasive paper that were mostly used for the aforementioned tests.

However, no limitation of the invention is to be seen therein. Additional tests with other materials and abrasive types have shown that the abrasive grain according to the invention yields advantages for other application areas as well that are therefore to be covered by the claims set forth below as contemplated by the invention. 

1. Abrasive grain comprising A1₂0₃ and Zr0₂ with a further content of titanium compounds in reduced form, between 0.5 and 10 wt. % thereof, expressed as Ti0₂; a content of Zr0₂ between 20 and 50 wt. %, with more than 85 wt. %, preferably not less than 90 wt. %, of the Zr0₂ being present in its tetragonal crystal form; an overall carbon contents between 0.03 and 0.5 wt. %; a content of raw material caused impurities of less than 3.0 wt. %, the content, if any of rare earths, calculated as oxides, amounting to less than 0.1 wt. %; and with a content of A1,0₃ between 50 and 80 wt. %; obtainable by melting a mixture of A1₂0₃, Zr0₂, Ti0₂ and SiO₂ and/or of raw materials containing the said oxides or precursors and necessary for producing the corresponding composition of the abrasive grain as stated above, in the presence of an excess of a reducing agent, and quenching the melt in such a way that the melt fully solidifies in fewer than 10 seconds:
 2. Abrasive grain according to claim 1 characterized in that the content of reduced titanium compounds expressed as Ti0₂ lies between 2 and 5 wt. %.
 3. Abrasive grain according to claim 1 characterized in that the share of tetragonal ZrO₂ modifications amounts to not less than 85 wt. %, preferably not less than 90 wt. %.
 4. Abrasive grain according to claim 1 characterized in that the share of raw material-caused impurities lies below 1 wt. %.
 5. Abrasive grain according to claims 1 characterized in that the share of additional Si compounds, expressed as Si0₂ lie between 0.2 and 0.7 wt. %.
 6. Abrasive grain according to claim 1 in that the ratio of A1₂0₃: Zr0₂ in the area of the eutectic system lies between 35 and 45 wt. % of Zr0₂ and 65 and 55 wt. % of A1₂0₃, in each case relative to the overall contents of A1₂0₃ and Zr0₂.
 7. Abrasive grain according to claim 1 characterized in that the abrasive grain possesses an extremely fine structure with dendritic eutectic domains of A1₂0₃ and ZrO₂ with a median cross section of fewer than 20 μm.
 8. Method for the production of an abrasive grain in accordance with claim 1 characterized in that a) a corresponding mixture of the raw materials is melted in the presence of reducing agents, b) the liquid melt is poured off and quenched, and c) after cooling down, the solidified material is crushed and treated for abrasive graining.
 9. Method according to claim 8 characterized by the fact that the liquid melt is quenched in such a way that it is completely solidified in fewer than 5 seconds, preferably in fewer than 3 seconds.
 10. Method according to claim 8 characterized [in that alum earth is used as raw material for A1₂0₃.
 11. Method according to claim 8 characterized in that baddeleyite, zircon sand and/or man-made zircon concentrates are used for Zr0₂.
 12. Method according to claim 8 characterized in that rutile is used as raw material for TiO₂.
 13. Method according to claim 8 characterized in that quartz, mullite and/or zircon sand are used as raw material for Si0₂.
 14. Method according to claim 8 characterized in that an Si0₂-containing raw material containing one or more oxygen compounds of the elements from the Al, Ti and Zr group, in addition to Si0₂, is used as raw material for Si0₂.
 15. Method according to claim 8 characterized in that coal in the form of petrol coke and/or graphite and/or anthracite is used as a reduction agent.
 16. Use of the abrasive grain in accordance with one or several of claims 1 through 7 in flexible and bound abrasives.
 17. Method of producing a flexible abrasive comprising dispersion of the grain of claim 1 on a flexible substrate and adhering the abrasive thereto.
 18. Method of producing a bonded abrasive comprising the steps of consolidating abrasive grain of claim 1 in a bonding matrix.
 19. A flexible abrasive product incorporating the abrasive grain of claim
 1. 20. A bonded abrasive product incorporating the abrasive grain of claim
 1. 